1. Technical Field
The present invention relates generally to circuitry for sensing a variable capacitance and, more particularly, to circuitry for sensing an output from a capacitive touch device sensor and converting that output to an analog and/or digital voltage signal.
2. Description of Related Art
A touch device includes a sensor surface and means for detecting whether an object (such as a finger, nail, pen, and the like) is either in contact with the surface (also referred to in the art as “touch detection”) or is approaching the surface (also referred to in the art as “proximity detection”). The touch device may be integrated with a display device to form a component commonly referred to as a “touch screen.”
The touch device is comprised, generally speaking, of a capacitance whose value is modulated by the presence of the touching or approaching object. A sensing circuit may be coupled to sense changes in the capacitance value and convert those sensed changes into a voltage signal. The generated voltage signal may then be further processed as desired and needed for certain applications. For example, the voltage signal may be converted by an analog-to-digital controller into a digital value, with that digital value being used to trigger certain control functions. The overall process is commonly referred to as capacitance to voltage sensing (or conversion).
Touch devices are not the only devices where a physical parameter can be monitored by sensing variation in capacitance value. For example, pressure sensors, movement sensors and accelerometers are well known in the art to utilize a capacitive sensing technique. Each of these devices requires some form of a capacitance to voltage sensing circuit.
Signal to noise ratio (SNR) is a critical factor in capacitive sensing type devices. In operation, it is well known that a noise component can be introduced on the sensing capacitance. It can be difficult in some cases to extract the signal of interest relating to the changed capacitance value (relating to the presence of the touching or approaching object) from the introduced noise.
Reference is now made to FIG. 1 which illustrates a prior art capacitance to voltage sensing circuit. The variable capacitance to be monitored is represented by a measurement capacitor Cm. A first plate of measurement capacitor Cm is coupled through a switch S1 to a force node F. A second plate of capacitor Cm is coupled through a switch S2 to a sense node S. Force node F is coupled through a switch S3 to ground and through a switch S4 to a reference voltage Vdd. The sense node S is coupled through a switch S5 to a capacitance to voltage sensing circuit 10 and through a switch S6 to a reference voltage Vcm. The reference voltage Vcm can be any desired value, and is commonly zero volts (or ground). The voltage sensing circuit 10 comprises an operational amplifier 12 having a first input coupled to the sense node S and a second input coupled to the reference voltage Vcm. An output of the operational amplifier 12 is coupled to the first input (and sense node S) through a holding capacitor Ch (and is further coupled to the sense node S through switch S5). A switch S7 is coupled in parallel with the holding capacitor Ch between the output of the operational amplifier 12 and the first input. The operational amplifier 12 and holding capacitor Ch function, as will be described in more detail below, as an integrator circuit. The output of the operational amplifier 12 is further coupled to the input of an analog-to-digital converter (ADC) circuit 14.
The switches S1 and S2 function as a switch matrix to selectively connect the voltage sensing circuit 10 to a selected measurement capacitor Cm in situations where the voltage sensing circuit 10 is a shared resource for multiple measurement capacitors Cm, as would be the case in a touch device having a matrix configuration. It will be recognized that in implementations were voltage sensing circuit 10 need not be a shared resource the switches S1 and S2 can be deleted or left in the closed position.
A switch control circuit 18 is provided to control the actuation of the switches S1-S7. The switches S1-S7 may comprise MOSFET type switches. The control circuit 18 operates in accordance with the multi-phase timing control operation in a manner to be described.
Operation of the circuitry in response to the control circuit 18 is as follows: During a first phase, switches S1 and S2 are closed to connect capacitor Cm to the force node F and sense node S. Switch S4 is closed to apply reference voltage Vdd to the force node F. Switch S6 is closed to apply reference voltage Vcm to the sense node S. Switch S7 is also closed. Thus, in this first phase, the measurement capacitor Cm and the holding capacitor Ch are both reset, and a voltage is applied across the measurement capacitor Cm which stores charge as a function of the applied voltage and the presence of the touching or approaching object.
During a second phase, switches S1 and S2 remain closed, but switches S4 and S6 are opened, while switches S3 and S5 are closed. Also, switch S7 is opened. Thus, the force node F is grounded, and the sense node S is coupled to voltage sensing circuit 10. This is referred to as a “forced switching” of the measurement capacitor voltage. In this second phase, the charge previously stored in the measurement capacitor Cm is transferred to the holding capacitor Ch by way of an integration process.
During a third phase, switches S1 and S2 remain closed, but switches S4 and S6 are closed, while switches S3 and S5 are opened. This isolates the measurement capacitor Cm and sense node S from the input of the operational amplifier 12. At this point, the ADC circuit 14 is activated to convert the voltage represented by the transferred charge, and present at the output of the operational amplifier 12, to an output digital signal 16. The analog-to-digital conversion will last for the duration of the third phase. The multi-phase process then repeats for the measurement capacitor Cm (or for other selected measurement capacitors in a matrix configuration by selectively controlling switches S1 and S2).
For a given measurement capacitor Cm, the voltage at the output of the operational amplifier 12 in the third phase is: Vout=Vcm+Cm/Ch*Vdd. To the extent the capacitance of the measurement capacitor Cm varies (due to object touching or proximity), then the variation in capacitance ΔCm will produce a variation in the voltage at the output of the operational amplifier 12 in the third phase: ΔVout=ΔCm/Ch*Vdd.
In the same manner it is possible to derive the effect on the output voltage due to the introduction of noise. Such noise could, for example, be introduced to the sense node S, and can be modeled by a noise capacitance Cn (or other applied noise voltage) coupled to sense node S. The change in output voltage at the output of the operational amplifier 12 is then given by: ΔVout=Cn/Ch*Vdd. The signal to noise ratio (SNR) is thus given by SNR=ΔCm/(2*Cn)*(Vdd/Vnpp), where Vnpp=the difference between the high value of the noise voltage and the low value of the noise voltage.
In an exemplary scenario, consider ΔCm=0.2 pF, Cn=0.5 pF, Vdd=1.8V and Vnpp=2V. This results in a signal to noise ratio SNR=0.18. If the input range is 3 pF (i.e., Ch=6 pF), the output swing is 900 mV for 3 pF and 600 mV for 2 pF. For a ΔCm=0.2 pF, the variation is only 60 mV relating to the presence of the touching or approaching object. However, the output peak-to-peak noise introduced by the noise source can be as much as 333 mV. In this situation, it is not possible (or is extremely difficult) to extract the wanted signal information out from the noise output voltage.
Again, SNR=ΔCm/(2*Cn)*(Vdd/Vnpp). As ΔCm and Cn are environment dependent variables, the only two parameters in the SNR equation that can be effectively controlled are Vdd and Vnpp. Thus, one solution to addressing the SNR issue is to boost the desired signal by using a higher reference voltage Vdd applied to the force node F of the measurement capacitor Cm. For example, a higher voltage in this scenario might be in the range of 20V (as compared to a lower voltage of 1.8V for Vdd as discussed above). The higher reference voltage Vdd will produce a higher desired signal value, and the SNR will improve proportionally to increases in desired signal magnitude.
There is a need in the art to provide a capacitance to voltage sensing circuit suited to operation at such higher reference voltage Vdd levels.